What's the lowest lumen rating that the eye can detect? Less than .003 lumens?

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What's the lowest lumen rating that the eye can detect? Less than .003 lumens?

As many of you have the Thrunite Ti in possession with .04 lumen rating have already discovered, it's much brighter than many would have thought. Just a couple years ago we had a thread based around the 2 lumen mark and others have followed, but what has not (at least to my knowledge) been verified is the absolute lowest lumen rating that one could possibly see by.

Just to be clear, the topic is not what's practical, as that is a variable, but what the lowest that a dark adapted eye of either most people, or even those with extremely sensitive eyes could see by, and at what distance.

My personal interest in this is to find what actual output level would make super-low output level fans like BeaconOfLight, myself and others finally satisfied. I've heard many times about lights that went so low as to be useless, from the Novatac 120, all of Henry's lights, the Titan and countless others. But every time I got my hands on those same lights I realized they were damn bright in context of a super-low output rating and recently my curiosity was once again piqued by the arrival of my programmable Calipsoii ring for my Surefire A2.

Doing a direct comparison so far with my hCRI Clicky and Ti the Calipsoii ring on one emitter is capable of doing what my awkward visual acuity would tell me is aproximately a quarter the output of the Ti, if not less, while the Clicky is nothing short of blinding at it's rated .07-.08.

One light that I know can easily go well below usable levels is the Spy series, but I still have no idea of what the actual lumen rating would be at the Spy or Calipsoii settings, and that's what I'm hoping to find out here.

My suspicion for some time now has been that at least down to .01 lumen, and quite possibly down to .001 is usable depending on distance, setting, general light sensitivity of user, coupled with comfort level at working in low light. I would naturally assume that night hikers, cavers and such would innately understand this topic, hope that all can benefit from the discussion, and moreover hope for some input from the integrating sphere/lux meter owners out there.

Just to be sure, even if the lumen scale is not the best for scientific purposes, it is what most lights are rated by at present and if in the future a better scale is used for this application, maybe we'll all benefit. For the time being, lumens it is, for the layman.

*As this thread goes along I'm going to try and keep updating the title to show the lowest visible lumen rating available.*

Last edited by kaichu dento; 01-07-2012 at 07:20 AM.

Marduke - Solitaire...I've seen matches which are brighter AND have a longer runtime.光陰矢の如し

Re: What's the lowest lumen rating that the eye can detect? Less than .01 lumens?

I wanted to try and elicit input from all the possibilities you've mentioned, partially out of a desire to find out what actual numbers would be for edge-of-blindness output levels, and also to finally satisfy some of the curiosity here as to what it actually meant when we see every lower lumen ratings from various companies, but never with much of a yardstick that is easily comprehended by the laymen among us.

The fact that the eye can do many things is one of the things that makes it so difficult to settle on an absolute 'best emitter, best light' for all, and of course the light source would be flashlights in normally useful ranges. Of course if someone wants to point out that their red or green or blue or neutral, cool, hCRI or whatever is more effective than another at given ranges, that may be interesting too, but the thing I was most hoping to find was someone willing to do some middle of the night, eyes completely dilated, integrating sphere testing.

What is the lowest output on a Titan and a ww Calipsoii ring with one emitter on it's lowest setting? The latter is almost low enough to satisfy me.

Marduke - Solitaire...I've seen matches which are brighter AND have a longer runtime.光陰矢の如し

I just received a Jetbeam RRT-0 with infinite variable ring. The low on it is, from what I've read on the reputable site I ordered it from is .003 lumens. If I were in a place I've never been Before(in the dark of course) this light on low would be no help at all. beyond maybe 2-4 feet.

Re: What's the lowest lumen rating that the eye can detect? Less than .01 lumens?

John, judging from your estimation of useful out to 2'-4', I'd say we can safely assume a level as low as .001 to be easily visible.

Mike, 1lumen at close range is blindingly bright when it's really dark, and I could easily detect the side of a house with the same lumen.

To tell the truth I didn't really expect much in the way of responses as there are only so many of us who truly appreciate extreme low levels, but since it's something that I care a lot about, I'll keep hoping for some more help in the quest to validate the lowest usable output level.

To clarify for those of you who've responded so far, if it's usable only at 1' then it's still usable. A few years ago I was walking along using the lowest setting on my D10 on a very dark night, and enjoying the reasonable light it cast on the road, but when I went to look at a sliver in my hand, I was shocked (dismayed) at how overly bright it was, and there was no turning it any lower.

Marduke - Solitaire...I've seen matches which are brighter AND have a longer runtime.光陰矢の如し

Re: What's the lowest lumen rating that the eye can detect? Less than .01 lumens?

Originally Posted by kaichu dento

As many of you have the Thrunite Ti in possession with .04 lumen rating have already discovered, it's much brighter than many would have thought. Just a couple years ago we had a thread based around the 2 lumen mark and others have followed, but what has not (at least to my knowledge) been verified is the absolute lowest lumen rating that one could possibly see by.

In short, answering such a question is problematic, I think. There are not only the issues that 127.0.0.1 brought up, but objectively speaking, the human eye (specifically scotopic or "rod" vision) is extremely sensitive, even down to the quantum level. A single photon is all it takes to get the retina to respond, although a few more within a short time span are required to trigger perception, as described in more detail here:

The scale of the amount of light we're talking about is so small that it's possibly "lost in the noise" at the scale of the lumen ratings of mass-produced flashlights.

By the way, many other creatures have adaptations that allow them to generally see better than we do in the dark, but if the question is absolute minimum sensitivity (i.e. able to see anything at all), then the difference, in absolute terms, cannot be that great.

Originally Posted by kaichu dento

My personal interest in this is to find what actual output level would make super-low output level fans like BeaconOfLight, myself and others finally satisfied. I've heard many times about lights that went so low as to be useless, from the Novatac 120, all of Henry's lights, the Titan and countless others. But every time I got my hands on those same lights I realized they were damn bright in context of a super-low output rating and recently my curiosity was once again piqued by the arrival of my programmable Calipsoii ring for my Surefire A2.

Those who claim that such levels of light, which to give you a sense of scale involves multiple trillions of photons per second, are useless were probably not using their eyes to the fullest. Few people these days ever do, outside of amateur astronomy, and even then only in exceptional cases (e.g. traveling to a dark site in the middle of a desert).

Originally Posted by kaichu dento

Doing a direct comparison so far with my hCRI Clicky and Ti the Calipsoii ring on one emitter is capable of doing what my awkward visual acuity would tell me is aproximately a quarter the output of the Ti, if not less, while the Clicky is nothing short of blinding at it's rated .07-.08.

A reasonable lower bound for the number of photons of visible light in 0.01 lumens is about 29 trillion per second (the real number is somewhat higher than that). Remember that humans can usually perceive (above the level of noise) as few as 9 photons entering the eye. While this is under extremely controlled conditions, and contrast is what really matters in practical use, the full scope of what we're talking about is immense. Under the right conditions, even 0.01 lumens could be considered "a lot" of light--we need to insert more zeros before we truly get to a minimal amount. For activities like reading, however, the required amount of light would be significantly greater.

Originally Posted by kaichu dento

One light that I know can easily go well below usable levels is the Spy series, but I still have no idea of what the actual lumen rating would be at the Spy or Calipsoii settings, and that's what I'm hoping to find out here.

How dark-adapted were you when you checked? It takes at least 30 minutes of complete darkness to attain reasonable dark adaptation, and a lot longer than that to find out just how little light you could get away with.

Originally Posted by kaichu dento

My suspicion for some time now has been that at least down to .01 lumen, and quite possibly down to .001 is usable depending on distance, setting, general light sensitivity of user, coupled with comfort level at working in low light.

I've seen vague references (I'll have to do more research) to 10^-6 cd/m² as being the minimum luminance necessary for human scotopic vision, but I don't know how this was determined. Roughly taking things like reflectivity into account, it means that from a few feet away 0.00001 lumens should supposedly be barely enough to illuminate a square meter area. Obviously you could get away with even less light than that if you illuminated a smaller area and/or viewed it from a shorter distance, though.

Originally Posted by kaichu dento

I would naturally assume that night hikers, cavers and such would innately understand this topic, hope that all can benefit from the discussion

Maybe cavers (under some circumstances) and amateur astronomers (although we can use a bit more light than absolute minimum as long as it's red, and already have specialized flashlights that do the job), but hikers tend to prefer plenty of light for safety reasons.

Originally Posted by kaichu dento

The fact that the eye can do many things is one of the things that makes it so difficult to settle on an absolute 'best emitter, best light' for all, and of course the light source would be flashlights in normally useful ranges. Of course if someone wants to point out that their red or green or blue or neutral, cool, hCRI or whatever is more effective than another at given ranges, that may be interesting too,

This is not merely interesting, but can be substantial at the extremes of human vision. One fact that is particularly relevant due to the prevalence of white LEDs in flashlights is that they all have a "valley" or "dip" in their spectra right about where human scotopic vision is the most sensitive. This means that for all I tried to convey above, maybe with LEDs specifically we need more total lumens in order to see at the lower end of the scale. In this respect, some--albeit not all--high-CRI LEDs are significantly better than others; they still have that dip, but it's a lot shallower. Tints such as cool white, neutral white, or warm white do not necessarily imply better or worse sensitivity at low light levels--it depends on the spectrum of the individual LED, not its overall color temperature. Monochromatic LEDs that cover this range--centered near 500nm--are often called aqua, turquoise, or NV-green (night vision green).

Originally Posted by kaichu dento

but the thing I was most hoping to find was someone willing to do some middle of the night, eyes completely dilated, integrating sphere testing.

By the way, dilation happens quickly and has less to do with dark adaptation than chemical processes within the retina, which take some time.

Re: What's the lowest lumen rating that the eye can detect? Less than .01 lumens?

Originally Posted by kaichu dento

. . . . A few years ago I was walking along using the lowest setting on my D10 on a very dark night, and enjoying the reasonable light it cast on the road, but when I went to look at a sliver in my hand, I was shocked (dismayed) at how overly bright it was, . . . . . .

???? That's exactly the point I was trying to raise - an amount of lumens can be very bright or very dim - UNLESS you specify a distance or area over which it's spread. Lumens is the TOTAL AMOUNT of light a torch puts out.

If you want to refer to a level of illumination that's independant of distance - you need to use Lux or Candela per square metre.

Re: What's the lowest lumen rating that the eye can detect? Less than .01 lumens?

I've lately been hooking up 10k potentiometers to 2 NiMH cells driving white LEDs. I am usually comfortable seeing to walk around my (newly moved into apartment with stairs) at what must be a drive current of (2.8/10000) or 0.28 mA. For a white LED, that is less than 0.1 lumens, right? Just shining a 20 degree spread white LED out ahead of me when I get up in the middle of the night.

Re: What's the lowest lumen rating that the eye can detect? Less than .01 lumens?

Originally Posted by AnAppleSnail

. . . . I've lately been hooking up 10k potentiometers to 2 NiMH cells driving white LEDs. I am usually comfortable seeing to walk around my (newly moved into apartment with stairs) at what must be a drive current of (2.8/10000) or 0.28 mA.. . . . .

???? To use ohms law to calculate current you need to use (2.8-VLED)/10000.

Originally Posted by AnAppleSnail

. . . . For a white LED, that is less than 0.1 lumens, right? . . . . .

LED efficiency for white LEDs takes a nosedive at low currents - and that's typically once you go below 1/10th of the rated current of the LED. Deriving output based on very low drive currents just doesn't work.

Re: What's the lowest lumen rating that the eye can detect? Less than .01 lumens?

Originally Posted by MikeAusC

???? To use ohms law to calculate current you need to use (2.8-VLED)/10000.

LED efficiency for white LEDs takes a nosedive at low currents - and that's typically once you go below 1/10th of the rated current of the LED. Deriving output based on very low drive currents just doesn't work.

My current calculation ignores the LED because I don't know its Vf. The stated Vf is 3.2v, and it lights clear down to almost 2v. Similarly, I have red LEDs that shine dimly on 1 alkaline AA. I know I'd have to use a light meter to really get a brightness number, but I can visually compare to lights that I think I know the brightness of.

Re: What's the lowest lumen rating that the eye can detect? Less than .01 lumens?

Great thread AnAppleSnail!

This is indeed an interesting question. I've been playing around with edge of detection lighting for a while and in my own experimentation (pure observation and probably not very scientific) I've found that color may have more to with what can be detected as providing light than actual lumens.

I would be interested if anyone has knowledge about the manufacture of LEDs about whether color makes a difference in efficiency or if it is all in the "eye" of the beholder?

Re: What's the lowest lumen rating that the eye can detect? Less than .01 lumens?

the limit of detection is even smaller if:

the object that is bouncing photons in your direction is moving rapidly across the field of view (about the speed of a tasty rabbit)

you are not looking right at it, rather at the very periphery where your eyesight is the most sensitive

we animals are designed to eat what moves... because it has much higher energy
density than plants and whatnot....and the eyes are optimized for spotting moving things
entering the field of view and not as optimized for straight-ahead high sensitivity or non-moving objects

Re: What's the lowest lumen rating that the eye can detect? Less than .01 lumens?

As has been mentioned the human eye is extremely sensitive and the answer to the original question I believe is that the eye is sensitive to any light. It probably isn't possible to come up with some level of light that isn't visible to the human eye.

Re: What's the lowest lumen rating that the eye can detect? Less than .01 lumens?

A reflectorless, lensless, pure flood would be useful for finding out your answer to your question. ZL H501w for instance.
The next part would be to either have faith in their published lumen levels or verify them yourself.
Then make a jig to hold a opaque card against another opaque card with a hole in it, braced against the face of your light (little paper rails would be needed along the sides to keep the opaque card aligned as it was slid progressively over the light).
It could be simply made with hot melt glue (for solid, removable foundation the bezel), tape (to hold the paper rails on either side of the base card) and two black spray painted playing cards so long as it could be adjusted easily in the dark and not slide around without finger pressure -for measurement later.
For multiple measurements in the dark a fine tipped pin or exacto knife could scribe a little line where the sides of the cards overlapped, with a good memory or a voice recorder you'd know the meaning of each mark.

Better still a fancier jig with the aperture stop mechanism from a camera held in front of the light! -but that's a whole nother level of commitment.

There would still be a inaccuracies since the measurement area is flat while the light is radially radiating but should be good enough for unpaid research.

Of course the rough expedient version of the experiment can be done without jigs or measuring fractions of a circle, in a few seconds, just cover the whole lens with the black paper, turn on low and pull the paper to one side till the light is effective at a given ranged task, then guesstimate percentage of light exposed.

Re: What's the lowest lumen rating that the eye can detect? Less than .003 lumens?

The OP poses a great and interesting question. Let me just confound things a bit. When I worked in technical theatre I occasionally had the task of training stagehands (and others in the technical space) how to use dim lights discretely. I found that often students had the required visual acuity to accomplish the task at hand but when they lacked the required concentration they couldn't see anyway.

My point is that there is another issue lurking around these perception issues, and that is that many persons have the visual acuity for a given setting but lack the concentration to be able to use what their eyes are seeing.

These are complicated issues and the answers are rarely as black and white as the would seem at first glance.

Re: What's the lowest lumen rating that the eye can detect? Less than .003 lumens?

now That is really interesting, let me restate it for myself and please correct what I misunderstand.

The human eye can potentially detect double or single digit quantities of photons, but the level of discrimination (signal/noise detection) needed would require massive processing power, i.e. attention/focus.
-so that's my translated copy, if I copied.

I read a book called "Better Eyesight Without Glasses" by Gregory Bates (and he mighta established the whole 20/20 thing, can't remember) but he distinguished between fine resolution eyesight of island people who could detect a dark spot on the horizon so well as to compete with the telescopes of the day, and jungle people who could detect tiny distinctions in visually complex jungle environments with mixed ranges of leaves, vines and whatnot.
The eye developing along with the imperatives of the watcher's concentration.
He also indicated that nearsightedness was virtually unheard of in loosely organized/low neurosis societies, and very common in highly organized (high tech societies). The subject of his book of course is attention/imagination/stress reduction and optical software/hardware reprogramming.

I was too unfocused to start doing the required exercises for the couple hours daily recommended, but i did them enough to see the benefit. It's like running, doesn't really matter unless you do it.

If you want to play around with a core concept of the book without reading the book, consider these propositions:
Acute eyesight is partially physical and partly perceptual.
Poor eyesight found in correlation with the Inability to vividly Imagine Blackness...

One of the primary exercises is to relax and imagine Blackness, liquid pools of ink, of oil, etc.
Another primary exercise is to rub your hands together until they are hot and rest them gently over your closed eyes while again imagining rich, vivid black.

Re: What's the lowest lumen rating that the eye can detect? Less than .003 lumens?

Wow, those are good examples.

It's the old story of 'use it or lose it.' For the most part modern man has machines for seeing things in detail. We do what we do. We not only don't see everything there is to see, we don't know what we don't see, either.

Re: What's the lowest lumen rating that the eye can detect? Less than .003 lumens?

I've got the thrunight scorpion V2 which advertises a .2 lumen "firefly". While sleeping in a hostel a while back with about 30 people sharing the room it was far more than enough light to navigate around with and frankly obnoxious to the others who were sleeping. To get up and go to the bathroom I covered the lens 95% and that was fine to see by. It's more than enough to in a tent to read by; the pronounced hot spot can be blinding and the spill coming from the side is plently.

The lowest setting on my varapower 2000 while running on 3 c's is signifiantly lower than that; it has no "claimed" lumens.

The next lowest I have is the .2 lumen setting on my Maelstrom G5. It's much, much lower than the .2 lumens setting no the varapower. It's actaully pretty tough to read by. It might be 10X lower than the .2 lumens on the varapower.

And the lowest (by far) that I have is the same varapower on the lowest setting when running it on 3 D's as oppossed to 4 C's. It's plenty bright to allow me to see the emittor itself though I can't see much by the emmitor. I find in useful in the absolute dark to leave it on, facing my diretion while sleeping to aid me in finding it. If it doesn't get covered, it's bright enough in complete dark to see it from 10' away. Not to be confused with seeing something 10' away which of course it does just fine with that on the next click of 32 settings up.

I'm sure others with the exact same lights may have diffferent results; I've found the consistancy of low lows are way off between brands and even between the same flashlight. I am sure my varapower .2 lumens is at least 10X as many lumens as the G5 for example.

Very interesting subject. From what little I have experienced, beam shape and tint play into what modes I use with lights. I am a neutral/warm fan, but it seems plain white seems to be more effective at the extreme lows.

I wonder how many folks can walk around the house with a Proton Pro on the lowest red. I think that level is best for reading a watch or something real close.

Re: What's the lowest lumen rating that the eye can detect? Less than .003 lumens?

Originally Posted by 127.0.0.1

the limit of detection is even smaller if:

the object that is bouncing photons in your direction is moving rapidly across the field of view (about the speed of a tasty rabbit)

you are not looking right at it, rather at the very periphery where your eyesight is the most sensitive

Good point and having thought a good deal about peripheral vision over the years, had never considered differences in light gathering when compared to central vision.

Originally Posted by gcbryan

As has been mentioned the human eye is extremely sensitive and the answer to the original question I believe is that the eye is sensitive to any light. It probably isn't possible to come up with some level of light that isn't visible to the human eye.

How right you are! LOL!

I'll make a distinction here since I didn't in the OP - it's usable light projected in the direction of the users interest that I'm concerned with and as such, any light source that won't at least make itself known at a distance of at least a few inches would be useless to me, even if it were visible when viewing the emitter alone.

Last night on the mountain my friend was checking to see how far my V10R and Clicky were at their lowest respective levels and even the .08 of the hCRI Clicky was able to project out to about 30', where I would hope to have it capable of going down to the level that it was usable only at 1' or less. Perfect for looking for something on the nightstand...

Originally Posted by Sub_Umbra

The OP poses a great and interesting question. Let me just confound things a bit. When I worked in technical theatre I occasionally had the task of training stagehands (and others in the technical space) how to use dim lights discretely. I found that often students had the required visual acuity to accomplish the task at hand but when they lacked the required concentration they couldn't see anyway.

My point is that there is another issue lurking around these perception issues, and that is that many persons have the visual acuity for a given setting but lack the concentration to be able to use what their eyes are seeing.

These are complicated issues and the answers are rarely as black and white as the would seem at first glance.

This is the most perfect type of response that I'd hoped to see here when I originally started this thread and it is exactly what I've been preaching to what mostly amounted to a brick wall for some time now - everyone can see at virtually the same levels - it's just whether or not their mind is capable of functioning at those levels.

I drive up the mountain on a regular basis at about 10mph in a Sus-V and they have a very low headlisht setting for stealth mode which is perfect for almost all my runs. The only time I turn them on full is when I want to more clearly assess the trail condition in order to speed up if possible, or slow down if necessary.

Most of the other drivers can't believe it when they first start, but once they give it a try they realize that it's way more than bright enough and it also offers a better opportunity for customers to see the sky, rather than just the trail ahead.

Originally Posted by eh4

It's like running, doesn't really matter unless you do it.

My feelings exactly!

Originally Posted by tbenedict

Very interesting subject. From what little I have experienced, beam shape and tint play into what modes I use with lights. I am a neutral/warm fan, but it seems plain white seems to be more effective at the extreme lows.

I wonder how many folks can walk around the house with a Proton Pro on the lowest red. I think that level is best for reading a watch or something real close.

That's one reason I like cooler tints at lower levels quite often but that would be and interesting side point to hear some experiences relating to tint and usability.

Marduke - Solitaire...I've seen matches which are brighter AND have a longer runtime.光陰矢の如し

Re: What's the lowest lumen rating that the eye can detect? Less than .003 lumens?

Originally Posted by kaichu dento

...it is exactly what I've been preaching to what mostly amounted to a brick wall for some time now - everyone can see at virtually the same levels - it's just whether or not their mind is capable of functioning at those levels...

It is a very real phenomena which I believe likely spans all of our senses. I have witnessed this firsthand in a number of disciplines. Since most people are visually oriented lack of concentration is very easy to see in sound operators, for example. (Sorry this doesn't read right, but it's true) In theatre sound operators fail to hear things under stress that they could easily hear if they were in a more relaxed, centered state. This also partially explains why deaf or nearly deaf sound operators continue to keep their jobs -- because though connected, their performance is not limited by their reduced acuity because they are compensating by doing a better job of concentrating.

Just as some doctors were told to take a few deep breaths to get the most info out of viewing an x-ray (an old relaxation technique) I used to tell technicians to take a few deep breaths before a difficult cue to try to relax; to center themselves to try to get themselves into a state where the low-level input would actually register (be it visual or audio).

Ask a police detective about this subject and they will probably tell you that when a witness is afraid or otherwise stressed they often don't remember all that may be expected of them.

Stress and concentration (or the lack thereof) most definitely affect the way we perceive and review lights. I think that Light Culture and concentration are factors we must consider every time we are asked for an opinion on some questions.

Whether it's, "...What light should I get to walk my dog," or, "...What light should I get to navigate the house at night," I have to constantly remind myself that these issues of acuity, concentration and stress are all things that we can not answer about another user and yet without that info the specs of the light in question, or even the opinion on that light from another user are close to worthless.

Re: What's the lowest lumen rating that the eye can detect? Less than .003 lumens?

In the ensuing months since the last posts were made we have come to accept that a light like the TC-R1 can be a common commodity and I would guess that the levels of light coming from the emitter at its lowest settings are much lower than the original number posted in the thread title.

Has anyone yet tried to measure the lowest outputs on a TC-R1?

Marduke - Solitaire...I've seen matches which are brighter AND have a longer runtime.光陰矢の如し

Re: What's the lowest lumen rating that the eye can detect? Less than .003 lumens?

Just read this thread for the first time.

I am not going to work out the calculations (call me lazy) but this should be an easy research exercise.

1) There are any number of resources, research papers, etc. that discuss not only the detection limits of the human eye, but the minimum light levels required to do certain tasks, descern objects, etc. Those limits will generally be expressed in terms of either candela or surface brightness depending on what is being referenced.

2) From either candela or surface brightness, you can work back using that number and an estimate of surface reflectivity, what the surface lux required is.

3) From the surface lux and an assumption of the beam pattern, you can calculate the needed lumens.

Would be interesting in a spreadsheet and I am sure others (well at least me) would be interested in the results. Swamped these days so I can't take on the exercise of finding the research papers again, but perhaps if someone researched the appropriate papers, someone else could take on the math (or me if you catch me on a good day).

Re: What's the lowest lumen rating that the eye can detect? Less than .003 lumens?

Pete - you know that I am a low lover like yourself. It is almost a must for any around the house EDC to have a super low low. I sold my LF2XT, as awesome as it was, because it was just way to bright on it's lowest setting.

I was made fun of at one point in the TCR1 thread because I was defending that even when the LED in the TCR1 was too low to be of any use (it's actually possible) it was still perfect for me. It's all good, I opted not to defend my position further as it was a useless battle, but I wasn't offended either.

I have three trits (two orange and one green) set into the base of my TCR1 with Norland and I've spent some time in a pitch black room comparing the lowness of the TCR1 against the three trits. I can say that I have it so that the three trits almost seemed like they were blinding. Although there is no way I could ever navigate a room just with the light of the three trits I can use it to see my hand or anything else within about a 1 foot range. I don't feel it has any practical application but it's just neat to know. Now I don't know if the advertised .003 lumens on the TCR1 is just a number or if there is any validity to it, but I can say that light can definitely go too low to be practical.

I agree with your OP that the lowest low on the HDS lights is ridiculously bright in a dark room, definitely can be too bright. The lowest level on the T1A is really just about perfect - I could use lower but really generally don't need to. I just acquired a TCR2 and while the lowest low on it is way below that of a HDS it is still brighter than the T1A and coulduse to be lower. The only other light I have that can fit this bill are my Spys - my level one setting is always a super dim one - for those who know the Spy UI I have my Tri-V level 1 set to blend the MC-E and XP-G and set it using position 6 and let it drop 38 blinks to find a perfect low.

I don't have any means to perform quantitative testing on the lumens level. But my qualitative judgement based on lights with specified low levels says that less that 0.01 lumens can still be practical for around the house navigation in pitch black.

Re: What's the lowest lumen rating that the eye can detect? Less than .003 lumens?

The main shortcoming on the LF2XT is the limited range - could go both higher (a little) and lower (a lot). But I'm still keeping mine for it's great UI and excellent beam pattern.

I've actually decided to use less trits, although they're all the rage these days. Too damn bright for use in conjunction with a couple of my lights that actually go low enough for my satisfaction. My TC-R1 is definitely one light that will go lower than I can possibly use it, which is very relaxing. They just went off the edge of the cliff for low level, and we finally have a light that can satisfy on both ends of the scale, top and bottom. The Spy's are another light that can go lower than anyone could wish for, but I found myself wishing that #3 position could go as low as #1 and #2. After programming the first two spots the way I wanted them, the third setting was too big a jump and I ended up having to compromise. In the end I was pretty well okay with it though.

Marduke - Solitaire...I've seen matches which are brighter AND have a longer runtime.光陰矢の如し